Colour measurement instruments used in colour development, pigment testing, or production control must provide the quality and quantity of data required with high reliability, serviceability, and speed of operation, independent of operating environment, and at reasonable cost. In a multi-plant operation, uniformity of testing equipment also becomes an important consideration. Whenever reflectance spectra are stored for use as numerical standards, a program of careful monitoring of the colour measurement instrumentation and of providing preventive maintenance at regular time intervals should be instituted. Without the associated support, a program of maintaining numerical standards will probably produce less than satisfactory results. Hence, a basic requirement of any colour measurement instrument is stability, i.e., it must give the same readings for an identical sample over a period of time.
All instruments consist of various optical and electrical components which all are subject to ageing processes. These ageing processes give rise to a gradual drift of the instrument readings over time. Eventually, the drift of an instrument can be traced back to a deteriorating detector element, to a distorted spectral power distribution of the lamp spectrum, or a shift in colour position of the calibration white tile. As a consequence of instrument drift it might be necessary to send it in to the manufacturer for service. However, when it is returned from recertification service the measured spectra of standards might differ from the previous average values and can not be made to coincide exactly with the gathered historical data. The change in reflectance measurements before and after repair must be considered as a design defect. This defect can be kept negligibly small by means of an efficient instrument profiling technology.
Methods have been devised, for example, in A. R. Robertson, Diagnostic performance evaluation of spectrophotometers, in Advances in Standards and Metrology in Spectrophotometry, ed. by C. Burgess and K. D. Mielenz, p. 277, Elsevier Science Publishers B. V. (1987) for diagnosing the performance of a spectrophotometer with respect to a reference instrument or reference data sets by measuring the spectral reflectance (or transmittance) factors of a set of special samples exhibiting a variety of spectral shapes and features. Different types of errors are separated by matching a composite error model to the experimentally observed spectral differences between sample and reference data sets. Any error whose effect on a particular sample can be expressed as a mathematical function of its spectral reflectance (or transmittance) factor can be included in the analysis.
For a daily quick-check of the performance of a colour measurement system the diagnostic tools described above are too time-consuming and complex for the operators of the equipment. Generally a system specialist will be responsible to thoroughly diagnose and document the performance of measuring instruments in regular intervals as, e.g., every six months. In addition to that process flows in industry require an efficient quick-test allowing the operator to validate the overall performance of instrumentation in possibly a single measurement step with easily interpretable results.
The major factors affecting the performance of colour measuring instruments are stability, photometric scale, wavelength scale or filter-fit, angle scale, and stray light. To define and validate these instrument scales and performance criteria, material standards have to be used. These material standards represent physical systems, whose properties depend on intrinsic variables as, e.g., the ambient temperature. Hence, the above list has to be extended to include as a further performance factor the material standards used.
It is recognised that the capability to make accurate measurements requires more than just the provision of a calibrated white standard. Additional quality assurance methods as, e.g., the provision of performance test samples, instructions for their use, the analysis of the results, diagnosis of any errors and instructions on how to remedy them, and retesting until satisfactory performance is achieved.
The performance of an instrument should be tested directly after it has been calibrated. It is convenient to test several aspects of performance at once. Obviously different material standards must be used for calibration and testing, since an error in standardisation or a defect in the standard could not be detected otherwise. Repeatability data for each instrument are routinely obtained using a set of dedicated stable coloured material standards as, e.g., the set of BCRA ceramic tiles (BRCA=British Ceramic Research Association). This set comprises 4 achromatic and 8 chromatic tiles. The achromatic (grey) tiles are useful to test photometric linearity, but are not useful to test errors associated with the wavelength scale. The eight chromatic tiles comprising brilliant red, yellow, and orange colour shades as well as saturated green, blue, and violet colour shades are useful in testing for wavelength errors. These chromatic colours exhibit steep slopes at various spectral ranges and serve to detect spectral shifts in the instrument's wavelength scale. Readings of these tiles provide the basis for determining the extent of instrumental problems and serve as reference point to which the instrument can be adjusted at each scheduled preventive-maintenance service call (at least every six months). As an instrument approaches the time for a preventive action, the colour difference calculated from the check-tile measurement begins to increase. On occasion, an increase in the check-tile's readings may result in the preventive maintenance to be performed ahead of schedule.
In case of modern goniospectrophotometers three different instrument scales have to be maintained and validated: the photometric scale, the wavelength scale, and the angle scale. Only the former two photometric and wavelength scales can be tested and adjusted by means of an appropriate set of solid colour standards, while a performance test of the latter angle scale requires gonioapparent reflectance standards. So far only solid colour standards as the set of BCRA ceramic tiles are commercially available and are widely used in industrial applications for validating the performance of colour measurement instruments. Since there is no accepted means in industry for checking the performance of the angle scale of goniospectrophotometers, any user of such colour measurement equipment has to design and produce his own proprietary standards for performance testing. Typically masstones of brilliant aluminium pigments exhibiting an extreme lightness travel are a good choice for the described purpose, since in addition to the high lightness flop the colour position of such paint formulations is almost free of thermochromism. A lightness drop of ΔL*≈180 units can easily be achieved for bright metallic colours between both viewing angles ε=15° and ε=45°, thus giving rise to a sensitivity of the order of ΔL*/Δε≈6/deg.
Special accessories are offered by instrument manufacturers or designed and build by users for standardised measurement of product samples. Such instrument holders allow the user to always position the goniospectrophotometer at the same spot at the sample surface and to fix and hold the instrument at the measurement position by exerting a certain amount of pressure on the top face of the instrument ensuring a close contact between sample surface and measurement head. Compared to a purely manual operation the variance of the measurement results can be significantly reduced when utilising such equipment. However, if instruments are integrated into robotic systems for contactless measurement of objects, slight mechanical instabilities or malfunctions may affect the orientation of the measurement plane and have an unintentional impact on the angle scale. Early recognition of geometrical malfunctioning by means of a suitable performance test would help to avoid down-time, time-consuming and costly complaints, and waste material.
Besides black and white calibration tiles, instrument manufacturers frequently also supply a special check tile with the instrument which can be used routinely for validating the performance of colour measurement instruments and correctness of colour measurement results. In general such tiles are formulated solid green or cyan colour shades and as such can only be utilised for testing the photometric and wavelength scales.
Stability of standards is a highly desired property, since they have to keep their measurement values over a long period of time (perhaps many years). Stability to environmental parameters other than time is likewise important. Standards should be stable to temperature, humidity, and light. This can not always be ensured. For instance brilliant red and yellow pigments are known to be extremely thermochromic. Such standards must be used at the temperature at which they are calibrated, and care must be taken that they do not change temperature due to absorption of energy while being measured. Other materials can be photochromic or hygrochromic.
In order to reduce the dependence of measurement values on position, uniformity of colour (reflected or transmitted) over the entire surface of the standard is important. This requirement can be extended to the lack of azimuth-dependence (orientation of the standard) and to independence of the state of polarisation of the incident light field. Considerations of uniformity also apply to the surface topography of standards. Generally they should be flat and highly polished, although in practice both ceramic tiles and porcelain enamels exhibit a slight surface pattern known as orange peel.
When analysing the reflectance properties of opaque gonioapparent coatings, for example, of colour- and/or effect-imparting base-coats as used in automotive coatings the reflectance level of high-angle readings is very low and the corresponding colour appears desaturated (less chromatic) and dark. Opaque gonioapparent colour shades can therefore only serve to validate the angle scale of a goniospectrophotometer. For a test of the other two photometric and wavelength scales further independent reflectance standards are required.
Therefore, there is still a need for a check-tile allowing a simultaneous quick-check of all three instrument scales of colour measurement instruments such as goniospectrophotometers.